518 research outputs found
Exogenous administration of gangliosides displaces GPI-anchored proteins from lipid microdomains in living cells
Exogenous application of gangliosides to cells affects many cellular functions. We asked whether these effects could be attributed to the influence of gangliosides on the properties of sphingolipid-cholesterol microdomains on the plasma membrane, also termed rafts. The latter are envisaged as lateral assemblies of sphingolipids (including gangliosides), cholesterol, and a specific set of proteins. Rafts have been implicated in processes such as membrane trafficking, signal transduction, and cell adhesion. Recently, using a chemical cross-linking approach with Madin-Darby canine kidney (MDCK) cells permanently expressing a GPI-anchored form of growth hormone decay accelerating factor (GH-DAF) as a model system, we could show that GPI-anchored proteins are clustered in rafts in living cells. Moreover, this clustering was dependent on the level of cholesterol in the cell. Here we show that incubation of MDCK cells with gangliosides abolished subsequent chemical cross-linking of GH-DAF. Furthermore, insertion of gangliosides into the plasma membrane of MDCK GH-DAF cells renders GH-DAF soluble when subjected to extraction with Triton X-114 at 4 degrees C. Our data suggest that exogenous application of gangliosides displaces GPI-anchored proteins from sphingolipid-cholesterol microdomains in living cells
Orbital currents and charge density waves in a generalized Hubbard ladder
We study a generalized Hubbard model on the two-leg ladder at zero
temperature, focusing on a parameter region with staggered flux (SF)/d-density
wave (DDW) order. To guide our numerical calculations, we first investigate the
location of a SF/DDW phase in the phase diagram of the half-filled weakly
interacting ladder using a perturbative renormalization group (RG) and
bosonization approach. For hole doping delta away from half-filling,
finite-size density-matrix renormalization-group (DMRG) calculations are used
to study ladders with up to 200 rungs for intermediate-strength interactions.
In the doped SF/DDW phase, the staggered rung current and the rung electron
density both show periodic spatial oscillations, with characteristic
wavelengths 2/delta and 1/delta, respectively, corresponding to ordering
wavevectors 2k_F and 4k_F for the currents and densities, where 2k_F =
pi(1-delta). The density minima are located at the anti-phase domain walls of
the staggered current. For sufficiently large dopings, SF/DDW order is
suppressed. The rung density modulation also exists in neighboring phases where
currents decay exponentially. We show that most of the DMRG results can be
qualitatively understood from weak-coupling RG/bosonization arguments. However,
while these arguments seem to suggest a crossover from non-decaying
correlations to power-law decay at a length scale of order 1/delta, the DMRG
results are consistent with a true long-range order scenario for the currents
and densities.Comment: 24 pages, 17 figures. Follow-up to cond-mat/0209444. (v2) Some
revisions in text, improved presentation. Minor changes in title, abstract
and reference
Integer Programming: Optimization and Evaluation Are Equivalent
Link to conference publication published by Springer: http://dx.doi.org/10.1007/978-3-642-03367-4We show that if one can find the optimal value of an integer linear programming problem in polynomial time, then one can find an optimal solution in polynomial time. We also present a proper generalization to (general) integer programs and to local search problems of the well-known result that optimization and augmentation are equivalent for 0/1-integer programs. Among other things, our results imply that PLS-complete problems cannot have “near-exact” neighborhoods, unless PLS = P.United States. Office of Naval Research (ONR grant N00014-01208-1-0029
Staggered flux and stripes in doped antiferromagnets
We have numerically investigated whether or not a mean-field theory of spin
textures generate fictitious flux in the doped two dimensional -model.
First we consider the properties of uniform systems and then we extend the
investigation to include models of striped phases where a fictitious flux is
generated in the domain wall providing a possible source for lowering the
kinetic energy of the holes. We have compared the energetics of uniform systems
with stripes directed along the (10)- and (11)-directions of the lattice,
finding that phase-separation generically turns out to be energetically
favorable. In addition to the numerical calculations, we present topological
arguments relating flux and staggered flux to geometric properties of the spin
texture. The calculation is based on a projection of the electron operators of
the model into a spin texture with spinless fermions.Comment: RevTex, 19 pages including 20 figure
Haldane-Gapped Spin Chains as Luttinger Liquids: Correlation Functions at Finite Field
We study the behavior of Heisenberg, antiferromagnetic, integer-spin chains
in the presence of a magnetic field exceeding the attendant spin gap. For
temperatures much smaller than the gap, the spin chains exhibit Luttinger
liquid behavior. We compute exactly both the corresponding Luttinger parameter
and the Fermi velocity as a function of magnetic field. This enables the
computation of a number of correlators from which we derive the spin
conductance, the expected form of the dynamic structure factor relevant to
inelastic neutron scattering experiments, and NMR relaxation rates. We also
comment upon the robustness of the magnetically induced gapless phase both to
finite temperature and finite couplings between neighbouring chains.Comment: 32 pages, 8 figures; published version includes additions discussing
the robustness of the magnetically induced gapless phase to ordering between
chains as well as the relationship between the spin-1 chains and spin-1/2
ladders in the presence of a magnetic fiel
Magnetization-plateau state of the S=3/2 spin chain with single ion anisotropy
We reexamine the numerical study of the magnetized state of the S=3/2 spin
chain with single ion anisotropy D(> 0) for the magnetization M=M_{S}/3, where
M_{S} is the saturation magnetization. We find at this magnetization that for
D<D_{c1}=0.387 the system is critical and the magnetization plateau does not
appear. For D > D_{c1}, the parameter region is divided into two parts D_{c1} <
D < D_{c2}=0.943 and D_{c2} < D. In each region, the system is gapful and the
M=M_{S}/3 magnetization plateau appears in the magnetization process. From our
numerical calculation, the intermediate region D_{c1} < D < D_{c2} should be
characterized by a magnetized valence-bond-solid state.Comment: 6 pages, 8 figure
CuSiO_3 : a quasi - one - dimensional S=1/2 antiferromagnetic chain system
CuSiO_3, isotypic to the spin - Peierls compound CuGeO_3, was discovered
recently as a metastable decomposition product of the silicate mineral
dioptase, Cu_6Si_6O_{18}\cdot6H_2O. We investigated the physical properties of
CuSiO_3 using susceptibility, magnetization and specific heat measurements on
powder samples. The magnetic susceptibility \chi(T) is reproduced very well
above T = 8 K by theoretical calculations for an S=1/2 antiferromagnetic
Heisenberg linear chain without frustration (\alpha = 0) and a nearest -
neighbor exchange coupling constant of J/k_{B} = 21 K, much weaker than in
CuGeO_3. Below 8 K the susceptibility exhibits a substantial drop. This feature
is identified as a second - order phase transition at T_{0} = 7.9 K by specific
heat measurements. The influence of magnetic fields on T_{0} is weak, and ac -
magnetization measurements give strong evidence for a spin - flop - phase at
\mu_0H_{SF} ~ 3 T. The origin of the magnetic phase transition at T_{0} = 7.9 K
is discussed in the context of long - range antiferromagnetic order (AF) versus
spin - Peierls(SP)order. Susceptibility and specific heat results support the
AF ordered ground state. Additional temperature dependent ^{63,65}Cu nuclear
quadrupole resonance experiments have been carried out to probe the Cu^{2+}
electronic state and the spin dynamics in CuSiO_3
The Inter-organizational Business Case in ES Implementations: Exploring the Impact of Coordination Structures and Their Properties
Developing the business case (BC) for an inter-organizational network is a major challenge. Factors like competition and differences in semantics between actors influence the stakeholders’ willingness to share information necessary for the BC development. In this paper we develop an exploratory framework showing the effect that coordination structure and project scope have on the development of a shared BC. We defined several coordination properties, such as competition, decision making location and decision power that mitigate this effect. We applied the framework in a case study where a BC is developed for an inter-organizational network. Our findings show that current BC development methods need to be re-stated and complemented by extra tools and interventions to support stakeholders in the inter-organizational specific setting
Competing Orders in Coupled Luttinger Liquids
We consider the problem of two coupled Luttinger liquids both at half filling
and at low doping levels, to investigate the problem of competing orders in
quasi-one-dimensional strongly correlated systems. We use bosonization and
renormalization group equations to investigate the phase diagrams, to determine
the allowed phases and to establish approximate boundaries among them. Because
of the chiral translation and reflection symmetry in the charge mode away from
half filling, orders of charge density wave (CDW) and spin-Peierls (SP)
diagonal current (DC) and -density wave (DDW) form two doublets and thus can
be at most quasi-long range ordered. At half-filling, umklapp terms break this
symmetry down to a discrete group and thus Ising-type ordered phases appear as
a result of spontaneous breaking of the residual symmetries. Quantum disordered
Haldane phases are also found, with finite amplitudes of pairing orders and
triplet counterparts of CDW, SP, DC and DDW. Relations with recent numerical
results and implications to similar problems in two dimensions are discussed.Comment: 16 pages, 5 figures, 4 tables. Revised manuscript; a misprint in Eq.
B3 has been corrected. The paper is already in print in PR
The Inter-organizational Business Case in ES Implementations: Exploring the Impact of Coordination Structures and Their Properties
- …